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Radiation Reactions: Dosimetry and Hot Atom Chemistry

• Dosimetry• Radiation Protection• Hot Atom Chemistry

Radicals are formed by the interaction of radiation with water

Radicals drive reactions

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Radiation Dosimetry• Units• Measurements• Neutrons and charged particles• LET• Dose Calculations

Dosimetry• Quantitative relation between specific measurement in a

radiation field and chemical and/or biological changes§ dose effect relationship§ caused by production of ionized molecules, atoms,

secondary electrons§ chemical changes, biological effects

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Units• Exposure

§ defined as amount of ionization in air caused by g and x-rays

§ roentgen (R) defined as amount of g or x-rays that produced 1 esu of charge in 1 mL of air at STP (1.293E-3 g)

§ new definition includes primary and secondary ionization

§ ∆Q/∆m; ∆Q charges of one sign, ∆m mass of air

refers only to EM radiation in air

1 R = 2.58x10-4 C/kg

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Dose Units• Absorbed Dose

§ energy absorbed per unit mass of target for any kind of ionizing energy

§ Gray (Gy) = 1J/kg§ in US; rad = 100 erg/g§ 1J/kg = 107 erg/103 g = 104 erg/g = 102 rad

• Absorbed dose is referred to as dose• Treated as point function, having a value at every position

in an irradiated object1 eV = 1.60E-19 J1 charge pair separation =1.60E-19 C1R = 2.58E-4 C/kg x 34 J/C = 8.8E-3 Gy

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Dose Equivalent

• Absorbed dose needed to achieve biological effect is different for different types of radiation§ Difference due to high versus low LET§ Dose equivalent compensates for this difference§ H (dose equivalent) = QD§ Q is dimensionless, has some different values§ Q=fn(particle, energy); 1≤Q≤20

à Q from NCRP Report 116§ uses LET (L) in keV/µm in water

• In soft tissue 1R produces 9.5E-3 Gy§ called rep (roentgen-equivalent-physical) no longer

used

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Dose Equivalent

• When dose in Gy, dose equivalent is Sv• When dose in rad, dose equivalent is rem (roentgen-

equivalent-man)• 1 Gy = 100 rad, 1 Sv = 100 rem• Particle type and energy should be explicitly considered• Distribution in tissue depends on isotope

§ I goes to thyroid§ Sr, Ra go to bone§ Cs, H go all over§ Metals go towards liver § Complexes can be released in kidneys

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QF Values for Various Types of RadiationRadiation QF

X and grays 1

Electrons and Positrons 1

Neutrons, E < 10 keV 3

Neutrons, E > 10 keV 10

Protons 1-10

Alpha Particles 1-20

Heavy Ions 20

Q Dependence on LET

LET (L) Q

(kev/µm in water)

<10 1

10-100 0.32L-2.2

>100 300/L^0.5

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Calculations• Alpha and Beta• Absorbed dose: D = AEavex1.6E-13J/MeVx1E3g/kg

=1.6E-10AEave (Gy/s)

A = conc. Bq/g,

Eave= average energy

Calculated dose of 1.2 E5 Bq of 14C in 50g of tissue

ßmax of 14C is 0.156 MeV

Eave≈ ßmax/3 ≈ 0.052 MeV

A = 1.2 E5 Bq/50g

D = 1.2 E5 Bq/50g x 0.052 MeV x 1.6E-10 = 20 nGy/s

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More Dose Calculations• Photons• µ/r is air energy absorption coefficient

= 0.0027 m2/kg for 60 keV to 2 MeV

D = 3.44E-17 CE/r2 (Gy/s)

C in Bq, E in MeV and r, distance from source, in m

gamma energy needs to normalized to %

Dose from 10000 Bq 38S at 0.1 m

95 % 1.88 MeV = 1.786 MeV

D= 3.44E-17 x 1E5 x 1.786 / 0.12

D = 6.14E-10 Gy/s

Need to consider average gamma energy

DCE

4r2

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Biological Effects Concepts• Linear Effect of Dose

§ Based on Atomic Bomb survivor data§ Mandated by law

à Any amount radiation above background is harmful§ Low level radiation effect not so clear

à No real evidence of health effects from any amount above backgroundà Some evidence of positive health effects with small increase in

radiation§ Recent efforts in low dose research

Time Event10-18 seconds Absorption of Ionizing Radiation10-16 seconds Ionization, Excitation10-12 seconds Radical formation, bond breakage10-12 to 10-6 seconds Radical reactionMin. to Hrs. Cellular ProcessesHrs. to Months Tissue DamageYears Clinical effectsGenerations Genetic Effects

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Radiation Protection• 3.6 mSv/y Typical background

radiation to US public(≈ 80% from natural sources)• 2.4 mSv/y Average dose to US

nuclear industry employees. (0.01mSv/y to public)

• 350 mSv in lifetime Criterion for relocating people after Chernobyl

• 50 mSv annual worker dose limit• 15 µSv to public from TMI in 50

mile radius• 0.4 mSv from dental x-rays• 1000 mSv as short term dose:

causes (temporary) radiation sickness.

• 5000 mSv as short term dose: would kill about half those receiving it within a month.

• 10,000 mSv as short term dose: fatal within days

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Regulations (US)• Dose limits vary from country to country• Directed towards occupational exposure

§ Public limits generally 10%• Rules set up by NRC (10CFR76.60) based on Atomic

Energy Act of 1954§ http://www.nrc.gov/reading-rm/doc-collections/cfr/

part020/à sets limits for isotopes, total exposure, procedures

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US Regulations

• Annual occupational limits§ 50 mSv total effective dose§ 0.5 Sv to any individual tissue or

organ(except eye)§ 0.15 Sv to eye§ 0.50 Sv to skin or to each extremity§ U intake limited to 10 mg/week

à Details at http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1201.html

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More regulationsDose to Public

• Total dose 1 mSv/year• not exceed 0.02 mSv in any one hour• may apply for 5 mSv/y• Compliance shown by 0.02 mSv in any one hour

or 0.5 mSv/y in uncontrolled area• For a nuclear waste repository 0.25 mSv/y• A risk of a detriment is associated with dose• Does not calculate number of people

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Probability Coefficients for Stochastic Effects

Detriment Adult Workers Whole Pop.

(1E-2/Sv) (1E-2/Sv)

Fatal Cancer 4.0 5.0

Nonfatal Cancer 0.8 1.0

Severe genetic

effects 0.8 1.3

TOTAL 5.6 7.3

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Calculations

What is probability of detriment from 2 mSv/y for 10 years to adult worker?

2E-3 Sv/y x 5.6E-2/Sv x 10 y = 1.1E-3

From maximum occupation dose for 30 years

50E-3 Sv/y x 5.6E-2/Sv x 30 y = 0.084

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Intake limits• Air and water

§ nuclide specific (include daughter)§ Class refers to lung retention (Days, Weeks, Years)§ Annual limits on Intake (ALI) derived from 0.05

Sv total dose or 0.5 Sv dose to an organ or tissue§ Derived air concentration (DAC) comes from ALI

DAC = ALI/(2000 hr x 60 min/hr x 2E4 mL/min)

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Intake Limits

• Public limited based on 0.5 mSv dose if concentrations continuously ingested or inhaled

Ingestion of 7.3E7 mL/y as basis of calculations

• Sewer Disposal§ Based on sewer water only water source, 5

mSv total dose per year

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Americium-241

AtomicNo.

Radio-nuclide Class

Table 1Occupational Values

Table 2Effluent

Concentrations

Table 3Releases

toSewersCol. 1 Col. 2 Col.

3Col. 1 Col. 2

OralIngestion

ALI(µCi)

Inhalation

Air(µCi/ml)

Water(µCi/ml)

MonthlyAverage

Concentration(µCi/ml)

ALI(µCi)

DAC(µCi/ml)

95 Am-241 W, all compounds

8E-1Bone Surf

6E-3Bone Surf

3E-12

- - -

(1E+0) (1E-2) - 2E-14 2E-8 2E-7

Isotope data found at: http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/appb/

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Laboratory usage• ALI and DAC basis of activities levels in the laboratory

§ http://rms.unlv.edu/radiological/Form%202%20-%20Risk%20Assessment%20and%20Control%20Guideline%20for%20RAM%20(2).pdf

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Hot Atom Chemistry• Also called Szilard-Chalmers process

§ Activity of I extracted from water and ethyl iodideà Precipitated at AgI

• Chemical reactions produced by nuclear transformation§ Neutron irradiation of ethyl iodide

à Iodine extracted into aqueous phase* 127I(n,g)128I

Ë Possible to produce specific isotope• Need to overcome bond energy

§ Neutron does not normally contain sufficient energy§ Gamma decay can provide suitable energy from recoil

à M is atom mass, E is gamma energy in MeV* Nucleus excited 6-8 MeV

M

EeVR

2537)(

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Hot Atom Chemistry• Bonds are broken due to reaction energy

§ Bond energies on the order of eV• Conditions needed

§ Bond of produced atom must be broken § Should not recombine with fragments§ Should not exchange with target molecule

à Slow kinetics § Separation of new species

• Halogens produced in this method§ CCl4

§ C2H2Cl2

§ C2H5Br § C2H2Br2

§ C6H5Br§ CH3 I

à Used to produce 38Cl, 80Br, 82Br, 128I

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Hot Atom Chemistry: Chemical Reactions

• Beta reactions can also be exploited§ TeO3

2-IO3- + e-

à Recoil is not quantized* Kinetic energy shared* E is maximum beta energy

(MeV)Ë Rmax(eV)=573E(E+1.02

)/MË 0.5 MeV in 100 amu is

about 4 MeV* Energy is distributed

Ë Translational, rotational, vibrational

* Bond usually not broken§ Internal conversion set atom in

excited stateà Rearrangement of electrons and

drive chemical reactionsà Separation of isomers

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Hot Atom Chemistry

• Conservation of momentum imparts recoil§ Solve based on momentum§ For M in amu and E photon

energy in MeV

à Er(eV)=537E2/M

§ Photon on the order of 7 MeV

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Review

• Dosimetry§ Calculations§ Units§ limitation § Influence of particles§ Measurements

• Hot Atom Chemistry§ Energetic processes

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Questions

• Compare DAC for isotopes of Pu and Cs• Perform a dose calculation for 1 mg internal

exposure of 210Po• Use DAC to evaluate experimental limits for

241Am• Calculate the dose from 500000 Bq of 241Am at

0.050 m• What are the principles of hot atom chemistry

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Pop Quiz

• Describe how ethyl iodide can be used in hot atom chemistry.